The increased drilling of directional oil wells requires that information related to bit orientation during drilling as well as data relating to the type of geological formation then being drilled be continuously transmitted to surface so that corrections can be made to the drill bit's orientation so as to guide the wellbore in the desired direction, and receive information as to the geologic formation being encountered.
When performing directional drilling, a measurement-while-drilling (MWD) mud pulser is commonly used to transmit such variety of measurements obtained downhole to the surface for processing. These measurements are transmitted as mud pulses, referred to as mud pulse telemetry. The mud pulser operates by modulating, downhole, pressure of the drilling fluid or ‘mud’ which is being pumped down the hollow drill pipe, in order to thereby transmit to surface, through the modulated pressure variations in the drilling mud, information relating to bit orientation and geologic formation.
Many designs of mud-pulsers have been used downhole, with varying success. One example of a direct current (DC) motor-operated mud-pulser uses a reversible electric motor together with Hall effect shaft sensors. The Hall effect sensors, mounted on the motor's stator, determine a position of the rotor relative to the stator and how (and when) to effectively commutate the motor (i.e., govern the respective energization of the respective stator windings of the DC motor so as to govern rotation of the DC motor). However, such Hall effect sensors are used not only for motor commutation, but are further used for sensing rotor revolutions and with revolution-counting circuitry are used for determining pilot valve position, in order to modulate the control valve and thus mud pulses being transmitted to surface.
Another design of mud-pulser has a brushless electric motor with a rotary-linear conversion system for governing a position of a pilot valve. However, this design does not have any rotor position sensor, such as Hall effect sensors, for commutation of the electric motor. This mud-pulser uses back emf voltages of the electric motor to both (i) commutate the electric motor, and (ii) further determine rotor position and number of revolutions, and thereby determine valve position. In such manner, the need for Hall effect sensors is avoided. Thus, a method for operating such electric motor uses the back emf to determine rotor position and involves keeping a count of phase transitions in a given motor direction, as a means of determining a position of a pilot valve relative to the completely opened position or completely closed position.
Where back emf is used to commutate a motor, the motor need to be turning at a minimum rate to generate sufficient back emf to be sensed. With sensorless commutation, at commencement of motor operation and when low motor rotations per minute (RPM), the back emf of the motor is low, which may result in less than perfect motor commutation.
Accordingly, there is a need to commutate an electrical motor with high precision and also to determine an angular position of the rotor of the motor.